Such sensors have a so-called shape effect and a so-called position effect. Shape effect means that test material not of exactly cylindrical cross-section generates signals of different strength depending on its transverse position in the measurement slot. Position effect means that material of arbitrary cross-section generates signals of different strength depending on its position between the measurement electrodes. In optical sensors, where the shape effect is particularly strong, an attempt is made to overcome the latter by scanning the test material using two mutually crossed light bundles. In capacitive sensors, in which the shape effect is substantially weaker but the position effect substantially stronger than in the optical sensors, attempts have been made to date to equalize possible deviations of the test material from the cylindrical cross-section by rotating the test material about its axis, and to achieve accurate positioning by complicated guides for the test material outside the measurement zone. With increasing demands on measurement accuracy, these methods tend to be increasingly unsatisfactory, and they also place high demands on the take-off means and transport means for the test material.